Process for producing a silicon melt

ABSTRACT

A process for controlling the amount of insoluble gas trapped by a silicon melt is disclosed. After a crucible is charged with polycrystalline silicon, a gas comprising at least about 10% of a gas having a high solubility in silicon is used as the purging gas for a period of time during melting. After the polycrystalline silicon charge has completely melted, the purge gas may be switched to a conventional argon purge. Utilizing a purge gas highly soluble in silicon for a period of time during the melting process reduces the amount of insoluble gases trapped in the charge and, hence, the amount of insoluble gases grown into the crystal that form defects on sliced wafers.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing single crystalingots having a reduced amount of crystal defects. More particularly,the present invention relates to a process for producing a silicon meltfor growing single crystal silicon ingots wherein the silicon meltcontains a very low amount of gases insoluble in silicon.

In the production of single silicon crystals grown by the conventionalCzochralski method, polycrystalline silicon in the form of granularpolysilicon, chunk polysilicon, or a mixture of chunk and granularpolysilicon is first melted down within a quartz crucible andequilibrated at a temperature of about 1500° C. Chunk polysilicon is apolycrystalline silicon mass which is generally irregular in shape, withsharp, jagged edges as a result of the fact that it is prepared bybreaking rods of polycrystalline silicon into smaller pieces; chunkpolysilicon typically ranges from about 2 centimeters to about 10centimeters in length and from about 4 centimeters to about 6centimeters in width. Granular polysilicon is a polycrystalline siliconmass that is generally smaller, more uniform and smoother than chunkpolysilicon as a result of the fact that it is typically prepared bychemical vapor deposition of silicon onto a silicon granule in afluidized bed reactor; granular polysilicon typically ranges from about1-5 millimeters in diameter and generally has a packing density which isabout 20% higher than chunk polysilicon.

As the polysilicon is heated and melted, an inert purge gas such asargon is continuously introduced over the crucible and silicon to removeunwanted contaminants from the melt area that are produced in and aroundthe melt during the melting of the polysilicon. After the silicon hascompletely melted and reached a temperature of about 1500° C., a seedcrystal is dipped into the melt and subsequently extracted while thecrucible is rotated to form a single crystal silicon ingot. During theearly stages of the melting process when the polycrystalline charge iscompletely or partially in the solid state, the purge gas may becometrapped in the polysilicon charge. The gas may be trapped between theindividual polysilicon charge pieces themselves, or between the chargepieces and the sides or bottom of the crucible and eventually becomeinsoluble bubbles in the melt which can be grown into the growingcrystal. Although most of the insoluble bubbles, such as argon bubbles,present in the melt are released into the adjacent atmosphere duringmelting and temperature equilibration, some remain in the silicon meltand can be grown into the silicon crystal, thereby producing voids inthe crystal.

While the problem of trapped gases occurs with all charge typesincluding chunk silicon, polycrystalline silicon, and mixtures thereof,the problem is particularly acute with charges formed from onlygranulated polycrystalline silicon; the granular polysilicon with itshigh packing density tends to insulate the bottom and side walls of thecrucible making it more difficult for insoluble gases such as argon toescape during the melting process. The purge gas, which hasconventionally been argon because of its low price and non-reactivenature, is highly insoluble in silicon. Because argon is highlyinsoluble in silicon, trapped argon gas in the melt forms small bubblesin the liquid silicon during melting. Many of the insoluble gas bubblescontained in the liquid melt rise to the surface or are carried to thesurface by convection and are released into the crystal growth gasambient and thus have no detrimental effect on the growing ingot. Asmaller number of the gas bubbles remain in the liquid melt throughoutthe pulling process and are grown into the crystal itself during growth.These bubbles, generally comprised of the insoluble argon purge gas,become trapped at the liquid-solid growth interface and cause largecrystal voids on the crystal surface. Such defects are generallycharacterized and detected on sliced silicon wafers as large pitsgenerally having a diameter of greater than about 1 micrometer. Thesepits are identified through laser scanning of polished wafers cut fromthe grown crystal. Such defects can effect 4% or more of wafers slicedfrom grown crystals and cause these slices to be unfit for grade onewafer product.

As such, a need exists in the semiconductor industry for a process ofpreparing a silicon melt for growing a single silicon crystal whereinthe silicon melt contains a very low amount of gases insoluble insilicon.

SUMMARY OF THE INVENTION

Among the objects of the present invention, therefore, are the provisionof a process for preparing a silicon melt containing a very low level ofgases insoluble in silicon; the provision of a process for preparing asingle silicon crystal containing a very low level of large crystalvoids; the provision of a process for producing a silicon melt whichproduces a high percentage of grade one wafers; the provision of asimple, cost-effective process which reduces the number of defects in agrown single silicon crystal; and the provision of a process forpreparing a silicon melt in which substantially all of the gas trappedin the silicon charge during the melting process is soluble in silicon.

The present invention, therefore, is directed to a process forcontrolling the amount of insoluble gas trapped by a silicon melt. Theprocess comprises first charging a crucible with polycrystalline siliconand heating the crucible to melt the charge. During the melting of thepolycrystalline charge a purge gas is flowed into the polycrystallinecharge. The purge gas has a mole fraction of at least 0.1 of gas havinga solubility in silicon of at least about 1×10¹³ atoms/cm³.

The present invention is further directed to a process for controllingthe amount of insoluble gas trapped by a silicon melt wherein a crucibleis first charged with polycrystalline silicon and the crucible heated tomelt the charge. A purge gas having a mole fraction of at least 0.1 of agas having a solubility in silicon of at least about 1×10¹³ atoms/cm³ isflowed into the charge during a heating phase and a melting phase of thepolycrystalline melting process. The heating phase comprises the timeperiod during the melting of the silicon before molten silicon is formedand the melting phase comprises the time period from the formation ofmolten silicon until the polycrystalline silicon charge is completelymolten.

Other objects and features of this invention will be in part apparentand in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the flow pattern of a purging gasduring the melting of a polycrystalline silicon charge in a crystalpulling apparatus.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, it has been discovered thatthe number of large crystal voids which form in silicon single crystalsduring a Czochralski growth process as a result of the trapping of gasesin the crystal can be significantly reduced or even eliminated byflowing a purge gas which has a high solubility in molten silicon into apolycrystalline silicon charge as the charge is being melted.Advantageously, the process of the present invention may be used in theformation of a mass of molten silicon in a crucible, i.e., a siliconmelt, from polysilicon charges comprising chunk polysilicon, granularpolysilicon, or a mixture of chunk and granular polysilicon.

In the process of the present invention, a crucible is charged withpolysilicon and then heated to form molten silicon. Throughout thecharge melting process during which the polysilicon charge is heated andmelted, a purge gas is directed into the crucible. FIG. 1 shows the flowpattern of purge gas inside a crystal pulling apparatus 2 during theheating and melting of the polycrystalline silicon. Purge gas 6 entersthe crystal pulling apparatus 2 through purge gas inlet 4 and flows downinto the crucible 8 which contains polysilicon charge 10 through purgetube 12. Crucible 8 is supported by support structure 22 and heated byheaters 24, 26, 28, and 30. Purge tube 12 directs purge gas directlyinto crucible 8 and polycrystalline charge 10. The purge gas 6 has aLaminar flow pattern throughout the crystal pulling apparatus 2 andcrucible 8 and carries contaminants produced during the melting of thepolycrystalline charge away from the crucible area. Purge gas 6 exitscrucible 8 containing polycrystalline charge 10 through open areas 18and 20 and exits the crystal pulling apparatus 2 through purge gasoutlets 14 and 16. The flow rates of the purge gases are generally setto achieve a pressure above the crucible of between about 1 and about 40Torr, more preferably between about 10 and about 30 Torr, and mostpreferably about 25 Torr.

In accordance with the present invention, the purge gas comprises a gashaving a relatively high solubility in molten silicon. Preferably, thepurge gas comprises a gas having a solubility in molten silicon of atleast about 1×10¹³ atoms/cm³, more preferably at least about 1×10¹⁴atoms/cm³, more preferably at least about 1×10¹⁵ atoms/cm³, still morepreferably at least about 1×10¹⁶ atoms/cm³, still more preferably atleast about 1×10¹⁷ atoms/cm³, and most preferably at least about 6×10¹⁸atoms/cm³ to ensure sufficient solubility of the gas into the siliconmelt. Such gases include, for example, nitrogen, chlorine, helium,hydrogen, and neon with nitrogen being particularly preferred. Compoundgases having the desired solubility such as NH₃ or HCl are also withinthe scope of the present invention. The purge gas may comprise a singlegas or a mixture of soluble gases, or a mixture of argon and a solublegas; if a mixture of argon and a soluble gas is used, it is generallypreferred that the mole fraction of the soluble gas in the purge gasmixture be at least 0.2, 0.4, 0.5, or even 0.6. More preferably, themole fraction of gas in the purge gas mixture is at least 0.7, 0.8, 0.9,or even about 1. Thus, for example, the purge gas may comprise a mixtureof argon and nitrogen (and/or other gases having a high solubility insilicon). Regardless of the gases selected, the source gases preferablyhave a purity of at least about 99%, more preferably at least about99.9%, and most preferably at least about 99.99%.

For purposes of the present invention, the polycrystalline chargemelting step of a crystal growing process may be considered to comprisetwo phases: the heating phase and the melting phase. The heating phaseof the melting process comprises the time period before molten siliconhas formed in the crucible, including the time period before any heat isapplied to the crucible, and the melting phase of the charge meltingprocess comprises the time period from the formation of the first moltensilicon until the polycrystalline silicon charge is completely molten.

In accordance with the present invention, the purge gas preferablycomprises a gas having a high solubility in molten silicon during atleast a part of the heating phase of the melting step of a crystalgrowing process. The heating phase of the charge melting step (i.e.,before any molten silicon has formed in the crucible) is the stage atwhich the trapping of gas between polysilicon particles or at thesidewall formation or bottom is most problematic; during this stage, thepurge gas may become trapped between polysilicon particles or along thesidewall formation or bottom of the crucible. As previously noted,insoluble purge gases trapped in these locations may inadvertentlybecome incorporated into the growing crystal; soluble gases, however,will tend to dissolve into the melt thereby eliminating the bubblebefore it can become incorporated as such into the growing crystal. Theuse of purge gases having a high solubility in silicon during theheating phase thus significantly reduces or eliminates the presence ofinsoluble gases in the molten silicon and, consequently, the likelihoodthat a single silicon crystal grown from the molten silicon will containcrystal void defects. For at least a fraction of the heating phase,therefore, it is preferred that the mole fraction of gas(es) having ahigh solubility in molten silicon in the purge gas be at least 0.2, 0.4,0.5, or even 0.6. More preferably, the mole fraction of gas(es) having ahigh solubility in molten silicon in the purge gas is at least 0.7, 0.8,0.9, or even about 1 for at least 20%, 40%, 80%, or even 100% of theheating phase; that is, before molten silicon has formed in thecrucible.

As the charge melting process continues, the melting phase begins andmolten silicon is formed in the crucible and a layer of molten siliconbegins to collect and grow along the bottom and sidewalls of thecrucible. As more silicon continues to melt, the level of molten siliconin the center of the crucible grows. During this time period, thetrapping of gases becomes less problematic as insoluble gases are lesslikely to be trapped between the crucible sidewalls or bottom and solidpolysilicon particles. Also, insoluble gas trapping between polysiliconparticles themselves becomes less problematic as the polysilicon chargebecomes completely molten as the solid polysilicon particles remainingin the melt become wetted by the liquid silicon making it difficult forinsoluble gas to penetrate between the particles and become trapped.Consequently, the benefit of including a gas having a high solubility inmolten silicon in the purge gas decreases. Nevertheless, some benefitmay be realized by including a gas having a high solubility in moltensilicon in the purge gas during the melting phase, i.e., the phasebetween the point in time at which some molten silicon is formed in thecrucible and the point in time at which the polysilicon charge is fullymelted. For at least a fraction of this melting phase and preferablyuntil at least the bottom of the crucible is covered by a layer ofmolten silicon, the mole fraction of gas(es) having a high solubility inmolten silicon in the purge gas is preferably at least 0.2, 0.4, 0.4 oreven 0.6. More preferably, the mole fraction of gas(es) having a highsolubility in molten silicon in the purge gas is at least 0.7, 0.8, 0.9,or even about 1 for at least 5%, 10%, 20%, 40%, 80%, or even 100% of themelting phase of the charge melting process.

When the melting phase of the charge melting step of a crystal growingprocess is complete and the polysilicon charge is fully melted, nofurther purge gas can become trapped between the polysilicon charge andthe crucible sidewalls or bottom, or between the polysilicon chargeparticles themselves. At this point, the purge gas can be switched to aconventional argon purge or other purge, without concern as to the purgegas solubility in silicon. If the preferred nitrogen purge gas is usedduring the heating and/or melting phases of the charge melting process,after the polycrystalline charge has fully melted and become molten itis generally preferred that the purge gas be switched to argon oranother purge gas to control the amount of nitrogen dissolved in themelt. In one embodiment of the present invention, preferably no morethan about 5×10¹² nitrogen atoms/cm³ are dissolved into the liquid meltduring melting. An excess incorporation of nitrogen into the moltensilicon can lead to the formation of solid nitride particles which maymake it difficult to grow dislocation free crystals.

In another embodiment of the present invention a soluble purge gas suchas nitrogen can be used during the heating and melting of the chargemelting step of a crystal growing process and during crystal growth toincorporate at least 1×10¹⁰ atoms/cm³, more preferably at least 5×10¹³atoms/cm³, or more, into the growing crystal. The nitrogen doped crystalis then sliced into silicon wafers and subsequently processed bychamfering, lapping, etching, polishing or similar conventionalprocesses to produce finished silicon wafers. After processing, thewafer is subjected to a heat treatment through the use of a rapidheating/rapid cooling apparatus to out-diffuse oxygen and nitrogen inthe surface layers to eliminate defects in the wafer. This process isknown in the art and fully set forth in EPO Patent No. 0942077.

Silicon melts prepared in accordance with the present inventionutilizing a purging gas which has a high solubility in silicon for aperiod of time during charge melting contain a significantly reducedamount of insoluble gas as compared to melts prepared with theconventional argon purge. About 4% of silicon wafers sliced from singlesilicon crystals grown from conventionally prepared melts have at least1 large pit making them unsuitable for grade one product. As such, forevery 1000 silicon wafers produced from conventionally prepared meltsabout 40 are not useable as grade one product. Silicon wafers slicedfrom single silicon crystals grown from melts prepared in accordancewith the present invention are substantially free from large pits. Asused herein, the term “substantially free from large pits” means thatthe resulting number of wafers containing at least one large pit isreduced by at least 50%, more preferably at least 90%, and mostpreferably 100% from the number of pits on wafers produced from asilicon melt using the conventional argon purging gas for the entiremelting process. As such, for each 1000 silicon wafers sliced fromingots grown from the silicon melts of the present invention, at least20 more wafers, preferably at least 36 more, and most preferably 40 morewafers are useable as grade one product. The resulting wafers aresubstantially free from large pits as the amount of insoluble gasestrapped in the melt and transferred into the growing ingot issignificantly reduced or eliminated through the use of the melt of thepresent invention. Therefore, a much larger percentage of wafers slicedfrom the single crystal will be suitable for grade one material.

In accordance with the present invention, the silicon melt preparedusing soluble purging gases during the melting of the polysilicon may beused in combination with the quartz crucible disclosed by Holder in U.S.Pat. No. 5,913,975. The crucible described in U.S. Pat. No. 5,913,975 isprepared by fusing the crucible in an atmosphere such as synthetic airwhich contains a very low level of argon. As such, the fused crucible isnot a significant contributor of insoluble gases into the silicon meltduring the melting of the polycrystalline silicon. The combination ofthe silicon melt of the present invention and the crucible disclosed byHolder in U.S. Pat. No. 5,913,975 may lead to a further reduction ofgases insoluble in silicon being incorporated into a growing siliconingot and, hence, the production of more grade 1 wafers per growncrystal.

The present invention is illustrated by the following example which ismerely for the purpose of illustration and is not to be regarded aslimiting the scope of the invention or manner in which it may bepracticed.

EXAMPLE

In this Example two separate 32 kilogram silicon charges comprised of100% granular polysilicon were each melted in a 14 inch diametercrucible in a Kayex-Hamco 3000 furnace and single crystal silicon ingotsgrown therefrom. To increase the probability of the formation of bubblesfrom insoluble gases, each crucible was coated on the inside side wallsand bottom with a barium carbonate devitrification promoter. The siliconingots were subsequently sliced into individual silicon wafers whichwere single side polished and inspected for large pits.

The first 32 kilogram charge of granular polysilicon was melted under aconventional argon purge flowing at a rate of about 32 slm and apressure of about 25 Torr. The polysilicon was melted and allowed toequilibrate at a temperature of about 1500° C. The melting andequilibration process took about 3.5 hours. After the polysilicon hadequilibrated, a 120 mm diameter crystal was grown to a length of about1200 mm on the first attempt. The crystal was subsequently sliced andground into 100 mm wafers which were single side polished and inspectedby laser scanning on a Model CR 80 Laser Scanner (A.D. Optical) forlarge surface pits greater than 10 micrometers in diameter. The laserscanning determined that 3.2% of the polished silicon wafers had one ormore pits having a diameter equal to or greater than 10 micrometers atthe polished surface.

The second 32 kilogram charge of granular polysilicon was melted under anitrogen purge at a flow rate of about 52 slm and a pressure of about 25Torr for about 1 hour until there was a continuous layer of liquidpolysilicon in contact with the sidewalls and bottom of the crucible.After about 1 hour, the purge was switched to a conventional argon purgeat a flow rate of about 32 slm and a pressure of about 25 Torr. Thepolysilicon was completely melted and allowed to equilibrate at atemperature of about 1500° C. The melting and equilibration process tookabout 3.5 hours. After the polysilicon had equilibrated, a 120 mmdiameter crystal was grown to a length of 1200 mm on the first attempt.The crystal was subsequently sliced and ground into 100 mm wafers whichwere single side polished and inspected by laser scanning on a Model CR80 Laser Scanner (A.D. Optical) for large surface pits greater than 10micrometers in diameter. The laser scanning determined that 0.7% of thepolished silicon wafers had one or more pits having a diameter equal toor greater than 10 micrometers at the polished surface.

The results of this experiment show that the silicon wafers sliced fromthe single crystal produced from the melt prepared utilizing a purge gashaving a high solubility in silicon had about 80% fewer large pits onthe polished surface.

In view of the above, it will be seen that the several objects of theinvention are achieved.

As various changes could be made in the above-described melt preparationprocess without departing from the scope of the invention, it isintended that all matter contained in the above description beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A process for controlling the amount of insoluble gas trapped by a silicon melt, the process comprising: charging a crucible with polycrystalline silicon; heating the crucible to melt the polycrystalline silicon charge; flowing a purging gas into the polycrystalline silicon charge as the polycrystalline silicon charge is melted, the purging gas having a mole fraction of at least about 0.1 of a gas selected from the group consisting of nitrogen and hydrogen.
 2. The process as set forth in claim 1 wherein the purging gas has a mole fraction of at least about 0.2 of a gas selected from the group consisting of nitrogen and hydrogen.
 3. The process as set forth in claim 1 wherein the purging gas has a mole fraction of at least about 0.5 of a gas selected from the group consisting of nitrogen and hydrogen.
 4. The process as set forth in claim 1 wherein the purging gas has a mole fraction of at least about 0.9 of a gas selected from the group consisting of nitrogen and hydrogen.
 5. The process as set forth in claim 1 wherein during at least 20% of a heating phase of the polycrystalline silicon melting the purging gas has a mole fraction of at least 0.1 of a gas selected from the group consisting of nitrogen and hydrogen, the heating phase comprising the time period during the polycrystalline silicon melting before molten silicon is formed.
 6. The process as set forth in claim 5 wherein the purging gas has a mole fraction of at least about 0.2 of a gas selected from the group consisting of nitrogen and hydrogen.
 7. The process as set forth in claim 5 wherein the purging gas has a mole fraction of at least about 0.5 of a gas selected from the group consisting of nitrogen and hydrogen.
 8. The process as set forth in claim 5 wherein the purging gas has a mole fraction of at least about 0.9 of a gas selected from the group consisting of nitrogen and hydrogen.
 9. The process as set forth in claim 1 wherein during at least 20% of a melting phase of the polycrystalline silicon melting the purging gas has a mole fraction of at least 0.1 of a gas selected from the group consisting of nitrogen and hydrogen, the melting phase comprising the time period from the formation of molten silicon until the polycrystalline silicon charge is completely molten.
 10. The process as set forth in claim 9 wherein the purging gas has a mole fraction of at least about 0.2 of a gas selected from the group consisting of nitrogen and hydrogen.
 11. The process as set forth in claim 9 wherein the purging gas has a mole fraction of at least about 0.5 of a gas selected from the group consisting of nitrogen and hydrogen.
 12. The process as set forth in claim 9 wherein the purging gas has a mole fraction of at least about 0.9 of a gas selected from the group consisting of nitrogen and hydrogen. 